Angew. Chem. Int. Ed. 51, 7981–7984 (2012)

Whereas nature chose water as the solvent to carry out the reactions necessary for life, chemists tend to choose organic media and even go to great lengths to make sure that they are working under dry conditions. One of the reasons for not using water is simple: most organic reagents are not very soluble in it. This property, however, can actually be used to accelerate reaction rates in what are known as 'on water' reactions. These can occur for certain transformations when water-insoluble reactants are vigorously stirred in water to give aqueous emulsions or suspensions. Such reactions are thought to occur at the interface between the immiscible phases, but they are not fully understood. For example, the complicated structure of emulsions makes it difficult to quantify the effects of important reaction parameters such as water surface area and surface-to-volume ratio.

Now, Wilhelm Huck and colleagues at Radboud University Nijmegen and the University of Cambridge, have used a fluidic approach to control the interfacial features of two model 'on water' reactions to better understand and quantify the role played by the water surface. The reaction set-up consisted of a cross-junction fed by tubes containing either water or reagents dissolved in toluene. Monodisperse plugs of toluene and water form at the junction and, by controlling the flow rate of the immiscible phases or changing the diameter of the tubing, Huck and colleagues could control the interfacial surface-to-volume ratio of the reacting plugs. Using this set-up they studied the cycloaddition reaction of quadricyclane with diethyl azodicarboxylate (DEAD), and the ene reaction between β-pinene and DEAD, reactions known from previous studies to exhibit 'on water' behaviour.

Huck and colleagues observed a linear relationship between the percentage conversion of reactants into products (as analysed by NMR spectroscopy) and the interfacial surface area between the plugs, strongly supporting a role for the surface in enhancing the rate. They also calculated a decrease in the activation energy of the reactions when 'on water' in comparison to in toluene, which supports a mechanism involving a hydrogen-bond-stabilized transition state.